The U.S. Army MICOM has conducted many investigations to develop Class 1.3 (&lt;69 cards in the NOL gap test) Insensitive Munitions (IM) minimum signature propellants for its near-term and mid-term tactical missile systems. These next generation propellants contain ammonium nitrate (AN) oxidizer in inert and energetic polymer binders. AN is of interest for its HCl free combustion products, low cost, minimum signature and its insensitivity. However, tradeoffs of performance for insensitivity are made for these AN propellants. AN propellant specific impulse (Isp) values are less than those of current propellants formulated with energetic nitramines cyclotetramethylenetetranitramine/cyclotrimethylenetrinitramine (HMX/RDX) whose theoretical Isps may range from 245-250 seconds. Typical Isps of AN propellants are 228-240 seconds at 1000 psi motor operating pressure.
Some other undesirable features of AN propellant formulations are poor burning properties (low burning rates, high pressure exponents, and high temperature dependency (PiK), and the AN phase change phenomena which can lead to cracked propellant grains during temperature cycling. MICOM has extensively investigated ways to improve these undesirable properties of AN propellants. In addition to energetic nitramines, various burning rate additives have been evaluated to improve AN propellant ballistic and performance properties. Numerous phase stabilizers have been evaluated to prevent AN phase changes during temperature cycling.
Previous efforts to enhance the ballistics and performance of AN propellants, while maintaining Class 1.3 and minimum signature characteristics, have proven to be a most difficult task. Improvement in one area usually translates to a loss in another. Previous work with elemental boron and boron compounds resulted in substantial gains in both propellant burning properties and performance, but with increase of propellant signature and sensitivity. AN propellant containing 0.5% boron failed minimum signature testing which requires visible transmittance of greater than 90 percent. Other additives such as the dodecahydrododecaborane salts (B.sub.12 H.sub.2.sup.-2) improved AN propellant burning properties, but with a reduction in Isp performance and poor motor plume signature characteristics.
To enhance the performance of AN propellants while maintaining minimum signature properties, small amounts (&lt;10%) of energetic solid nitramines such HMX, RDX or CL-20 are typically added. Fuel additives such as HMX, or RDX can increase propellant sensitivity to class 1.1. (greater than 69 cards in Naval Ordnance Lab (NOL) gap test).
A fuel additive which does not adversely affect propellant sensitivity, or minimum signature, but which can also improve performance Isp, would be desirable for AN propellants.
Therefore, an object of this invention is to provide a fuel additive that enhances AN propellant performance while not adversely affecting propellant sensitivity or signature of the AN propellant.
Another object of this invention is to provide a fuel additive which is useful with AN propellant with inert or energetic binder systems for increasing burning temperature of the AN propellant and thus enhancing combustion efficiency.